Peak power trackers have been used for tracking power delivered to a load from a power source. Peak power trackers are disclosed in U.S. Pat. No. 5,493,204 issued to Caldwell on Feb. 20, 1996, and in U.S. Pat. No. 4,794,272 issued to Bavaro on Dec. 27, 1988. These maximum peak power trackers can be applied to solar arrays providing power to a load where the peak power trackers track the power being delivered from the solar arrays and adjusts operating parameters to maximize the amount of power delivered from the solar array for powering the load. Shared bus current sharing for current mode DC-DC converters is disclosed in U.S. Pat. No. 6,009,000 issued to Siri on Dec. 28, 1999. The shared bus is used for equalizing power delivered through a plurality of converters that convert solar array power into regulated power for powering the load.
The apparatus of Caldwell fails to operate consistently over wide operating ranges depending on the solar array voltage and the current power operating point. The solar array voltage is detected by capacitive differentiation for controlling the duty cycle of a pulse width modulated control signal. Normally, as the solar array voltage increases, the capacitive differentiation voltage increases producing an increase in a pulse width modulation duty cycle increasing the power operation point until passing the peak power point. As the solar array voltage decreases, the capacitive differentiation voltage decreases causing a decrease in the duty cycle providing a decrease in the power operating point. The duty cycle increases and decreases to operate the apparatus dithering about the peak power point. The apparatus fails to function when the solar array voltage has settled at a low voltage when the apparatus operates below the peak power point in a low voltage trapped state. The capacitive differentiation method cannot absolutely detect the slow changes in the solar array voltage after the solar array voltage becomes steady far below the peak power point on the lower voltage side of the peak power point in a power versus array voltage curve. The apparatus may fail to operate at the peak power point when the solar power initially increases from zero after a black out. The solar array voltage initially starts far below the peak power point level on the lower voltage side of the peak power point in the power versus voltage curve profile. In the trapped state, when the solar array voltage changes slowly, the capacitive differentiation method may fail to detect small voltage changes. When the array voltage is low, increasing the duty cycle draws additional current from the solar array tending to further reduce and collapse the array voltage into the low voltage trapped state.
The apparatus of Caldwell also has inherent instability. The apparatus may not function at or near the specific peak power point when the solar array has a high amount of voltage ripple as the apparatus dithers about the peak power point. As the solar array characteristics widely change due to aging and environmental factors. The peak power point and amount of ripple also changes significantly over varying conditions. The Caldwell apparatus does not ensure that the solar array voltage ripple around the peak power point can be controlled to be negligible as compared to the average value of the operating array voltage. Because the array source and load conditions vary, the array voltage ripple around the operating value is unpredictably large. Due to lack of a precise control to limit the array voltage ripple, a large filtering capacitor reduces the ripple, but a large filtering capacitor causes slow changes in the solar array voltage undetectable by capacitive differentiation. A large amount of unpredictable ripple can not be effectively reduced by a fixed value capacitor. Hence, the apparatus cannot sustain the stability of the array voltage within a predetermined ripple amplitude.
Bavaro teaches a peak power tracker that is applied to a stand-alone single DC-DC converter. The use of several power converters could be used to increase the amount of power delivered but would necessarily require additional control circuitry, and, the power delivered to the load may be unequal amongst the converters. Also, the peak power tracker uses a dither signal having a predetermined dither frequency operating in the presence of differing operating conditions. The use of the dither signal avoids initial low voltage trapping. The peak power tracker also uses a second order band-pass filter to detect the converter output current at the dither frequency. The dither signal is compared to the output current signal for controlling the peak power tracker. In practice, a second-order band-pass filter may not be precisely tuned to a center frequency at the dither frequency resulting in operation off the peak power point.
A slow varying control voltage signal modulates a pulse width modulated signal for controlling the DC-DC converter. The dither signal is coupled with the solar array voltage so that very small changes in the solar array voltage can be detected for controlled operation at the peak power point. However, small fluctuations of the pulse width modulation control signal does not guarantee insignificant ripple of the solar array voltage at all conditions under the sun illuminations because the solar array voltage is not effectively regulated over varied amounts of sun illumination and temperature. The peak power tracker cannot regulate the array voltage ripple to a predictable amplitude. The peak power tracker does not regulate the array voltage disadvantageously resulting in unregulated array voltage ripple. The slow pulse width modulation control signal has only two states, increasing and decreasing states and fails to provide regulated solar array voltages during steady state conditions with reduced ripple. The solar array voltage ripple can disadvantageously cause excessive and uncontrolled and imprecise operational dithering about the peak power point. These and other disadvantages are solved or reduced using the invention.